With reference to attached FIG. 1, in a CZ method-type single crystal pulling apparatus for growing and raising a single crystal silicon ingot (9) from a polycrystal silicon melt (10), a silicon seed crystal (12) fixed at the lower end of a pull wire (11) (or, alternatively, supported by a pull shaft) is dipped slightly in the silicon melt contained in a synthetic quartz crucible (4) provided in a chamber (2), and is pulled up at a predetermined rate while being turned about its axis by means of the pull wire, whereby a silicon single crystal rod is grown and raised.
In this kind of single crystal pulling apparatus, a chemical reaction takes place between the quartz crucible and the Si melt to generate SiO, which is dissolved in the Si melt. A part of the SiO evaporates from the melt and mixes in the atmosphere within the chamber, and other part stays in the Si melt.
When the SiO in the atmosphere builds up in the chamber, the normal growth of the single crystal is seriously affected. In order to prevent this phenomenon, the chamber is kept under reduced pressure by means of a vacuum pump (18) and, at the same time, it is supplied with an inert gas such as argon gas in order to maintain the inertness of the intrachamber atmosphere. Normally, therefore, the chamber is filled with an inert gas and the pressure in the chamber is kept at the level of ten-odd Torr.
As the pulling of the single crystal proceeds, the amount of SiO gas generated in the chamber increases to thereby push up the intrachamber pressure, which in turn causes stagnation of the flow of the inert gas. In order to prevent this phenomenon, a vacuum gage is attached to the chamber to detect the intrachamber pressure; and based on the result of the pressure measurement, the opening degree of a flow control valve which is provided in the exhaust system is controlled in a manner such that the intrachamber pressure is always constant or changed arbitrarily [Japanese Kokai No. 61-117191 (1986)].
Now, turning our attention to the oxygen concentration of a single crystal, it is known that as the pulling of the single crystal proceeds and the level of the Si melt lowers, the amount of SiO in the Si melt, which virtually determines the oxygen amount in the melt, decreases, and this results in a gradual decrease in the oxygen concentration in the growing single crystal ingot. Consequently, the axial uniformity in the oxygen concentration is lost, and the tail end portion of the ingot may end up with an oxygen concentration less than the minimum allowable value.
In the conventional technology, only the flow of the inert gas was controlled, and as such it was not possible to solve this problem of uneven oxygen concentration. But if the oxygen concentration of the single crystal is to be controlled with a sufficiently high precision, an extremely complicated control of the intrachamber pressure must be conducted with respect to the progress of the single crystal pulling operation.